Femtosecond time-resolved photo-stimulated desorption from ionic crystals

We have used the pump–probe technique to measure the positive ion yield, from ionic crystals, as a function of time delay between two femtosecond laser pulses. The two-pulse technique allows direct observation of solid state and surface dynamics on a femtosecond timescale. We find the ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between pulses. For example, the Mg+ desorption yield displays three distinct features; a coherence peak followed by a fast rise and decay features. In contrast, the yield of K+ from KBr displays only the coherence peak and picosecond decay features. The observed ion detection thresholds suggest that, although the nanosecond laser ion desorption mechanism may be dominated by defect photoabsorption, significant electron–hole pair production may contribute to the femtosecond laser desorption mechanism. By determining the ultrafast time-dependence of positive ion emission, we hope to reveal the mechanism of laser ion desorption for both regimes.